The present invention relates to the field of treating hydrocarbon-contaminated drill cuttings and, in particular to the field of thermal desorption treatment of hydrocarbon-contaminated drill cuttings.
Environmental regulations governing the use of oil-based drilling fluids have been tightened, especially for offshore drilling operations, because of potential adverse effects of discharged drill cuttings on the environment. Specifically, oil-based drilling fluids typically have very poor biodegradability in anaerobic conditions, such as those found in deep seawater. Accordingly, piles of cuttings, whose physicochemical properties do not significantly change, if at all, under such anaerobic conditions, build-up on the seafloor forming potentially environmentally harmful deposits. Also, some drilling fluids have high levels of aromatic hydrocarbons that could have potential adverse toxicity issues.
Because of shortcomings in off-shore treatment of drill cuttings, drill cuttings are sometimes collected and transported to shore for treatment and disposal. This increases the risk of accidental release of drill cuttings into water when transporting from the rig to the shore for on-shore treatment. Since offshore drilling rigs offer limited space, especially for storage of drill cuttings, efforts have turned to effective off-shore treatment of drill cuttings. But one of the disadvantages of off-shore cuttings treatment is the limited space available for equipment.
A number of known processes use a fluidized bed for vaporizing contaminants from solids. However, as discussed below, most of the processes produce significant quantities of fine particulates in the gas stream exiting the fluidized bed. Particulates pose problems in recovery of liquids and in off-gas released to the atmosphere. Therefore, many of the processes discussed below require an extensive dust collection system, such as a cyclone or baghouse filter, for removing particulates.
WO00/49269 (McIntyre, Aug. 24, 2000) describes a thermal desorption process in which drilling fluid vapors are thermally desorbed from drill cuttings. Hydrocarbon-contaminated drill cuttings are fed to a pressurized desorption chamber where a hot heating gas (e.g., 400-600xc2x0 F. (204-316xc2x0 C.)) is pumped into the chamber to heat the drill cuttings by convection. A mixture of drilling fluid vapors and heating gas is discharged through an overhead vapor outlet and cleaned drill cuttings are removed through an underflow cuttings outlet.
The gas mixture is preferably processed in a cyclone to remove fine particles entrained in the gas. The gas mixture is then condensed to recover drilling fluid vapor in liquid form for recycling to a drilling fluid storage and circulating system.
U.S. Pat. No. 5,882,381 (Hauck et al., Mar. 16, 1999) also describes a thermal desorption system for treating hydrocarbon-contaminated solids, in this case a vacuum thermal desorption system. An inert gas generator is used to maintain low O2 (below 7%) to prevent combustion in the process gas stream. The inert gas is fed to a fluidized bed at a temperature in a range from 600 to 1,600(F (316-871xc2x0 C.) to vaporize the contaminants.
The process gas exiting the fluidized bed contains entrained solids that are removed in a high temperature baghouse filter, such as a pulse jet ceramic filter dust collector. The gas stream exiting the baghouse filter is then treated in a pre-cooler and a condenser to remove any remaining particulate matter, water and contaminant.
U.S. Pat. No. 4,778,606 (Meenan et al., Oct. 18, 1988) relates to a process and apparatus for treating a polychlorinated biphenyl (PCB) contaminated solid. A contaminated sludge (5 to 90% H2O) is contacted with very hot air and combustion gases in a separator at a temperature of 850 to 2,500xc2x0 F. (454-1,371xc2x0 C.) The separator dries, classifies and conveys the sludge in a continuous operation. In the lower portion of the separator, partially dry particulates are fluidized to vaporize contaminants. Fine particulate matter is entrained in the gas flow out of the separator and fed to a cyclone separator.
Any particulate matter containing excess contaminant may be returned to a mixer upstream of the separator for recycling. The mixer mixes the dried particulate matter with the incoming sludge for feeding to the separator.
Meenan et al. suggest that, if desired, additional material such as clean water or chemicals can be added to the sludge in the mixer/feeder to provide a sludge having a predetermined percentage (e.g., 50% by weight water) or to disinfect or otherwise treat the sludge in the mixer.
Schattenberg (DE 36 04 761 A1, Aug. 20, 1987) also describes a thermal desorption for treating hydrocarbon-contaminated soil using a rotary tube or fluid bed. An inert carrier gas, such as nitrogen, is used for heating the soil to the boiling temperature of the hydrocarbon contaminant (e.g., 400xc2x0 C. (752xc2x0 F.)). Nitrogen, water vapor and vaporized hydrocarbons flow out of the rotary tube or fluid bed through a de-duster for separating particulates and then through a distillation tower for separating water and oil.
None of the above-mentioned processes describe or suggest treating the solids prior to thermal desorption in a manner to reduce the particulate discharge or to increase the particle size.
Weitzman (U.S. Pat. No. 5,200,033, Apr. 6, 1993) suggests using a binder in a solidification/stabilization process. Weitzman""s thermal desorption process uses a thermal contactor with electric or fluid-heated walls. Contaminated solids are agitated and moved through the combustor by steam jets, air jets, mechanical rakes, ploughs or arms. The wall temperature increases downstream in the direction of the solids movement to heat the solids and release volatile components. A purge gas, such as a non-condensible gas or superheated steam, is used to purge the volatile components released from the solids.
Binders may be added to stabilize and solidify the contaminated solids. Suggested binders include Portland cement, pozzolanic materials, fly ash, cement kiln dust, lime kiln dust, quicklime, calcium hydroxide, calcium oxide, magnesium compounds, sodium hydroxide, and soluble silicates. The binders may be fed separately into the chamber or premixed with contaminated soil.
Gases from the chamber are condensed to remove contaminant and water vapor and then passed through a particulate collection device (e.g., electrostatic precipitator, scrubber or fabric filter). Weitzman recognized that many types of solids will cake on hot surfaces such as the walls of the contactor. Accordingly, he provides a series of scrapers or rakes to scrape the walls of the contactor.
But drill cuttings are particularly prone to caking when heated due to the nature of the solids and the drill fluids. While processes like Weitzman""s can scrape the walls of the thermal contactor to deal with caking, fluidized bed processes are not conducive to such devices. Also, when caking occurs, a solid external layer traps hydrocarbon contaminants inside the cake, Resulting in ineffective treatment. Therefore, those skilled in the art have avoided adding additional components to the contaminated solids that might cause further caking.
On the other hand, thermal desorption processes and, in particular, fluidized bed processes, produce fine particulates that are not easy to deal with, especially when there are space limitations.
According to one aspect of the present invention, there is provided a process for removing hydrocarbon contaminant from drill cuttings generated in an oil drilling operation, comprising:
i) mixing drill cuttings containing a hydrocarbon contaminant with an agglomerant to produce a pre-treatment mixture;
ii) heating the pre-treatment mixture at a temperature effective to vaporize the hydrocarbon contaminant of the drill cuttings, under a condition in which drill cuttings particles that are normally vapor entrainable are agglomerated by the agglomerant, and caking of drill cuttings is inhibited;
iii) recovering drill cuttings having a reduced content of the contaminant, and
iv) recovering vaporized hydrocarbons having a reduced content of vapor entrainable particles.
According to a particular embodiment of the invention, there is provided the process for treating drill cuttings contaminated with at least one hydrocarbon, comprising the steps of:
(a) providing hydrocarbon-contaminated drill cuttings with a first particle size distribution having a first median diameter;
(b) mixing the hydrocarbon-contaminated drill cuttings with an agglomerant to produce a pre-treatment mixture;
(c) establishing a pre-treatment total liquid content in the pre-treatment mixture in a range from about 5 wt. % to about 20 wt. %, based on the total weight of the pre-treatment mixture;
(d) agitating and heating the pre-treatment mixture at a temperature sufficient to vaporize substantially all of the hydrocarbon while agglomerating vapor entrainable particles of the drill cuttings to form agglomerates; and
(e) recovering treated drill cuttings with a second particle size distribution having a second median diameter greater than the first median diameter, the treated drill cuttings having a residual hydrocarbon content of less than or equal to about 3 wt. %, based on the total weight of the treated drill cuttings.